In radar or ultrasonic filling level sensors working on the pulse echo principle, the filling level is determined from a certain echo that can be detected in the filling level envelope curve as representing the filling level. Thereby, the filling level envelope curve comprised of individual echoes is sampled by an analog-to-digital converter whereby the received filling level envelope curve is made available to a microprocessor or microcontroller for further processing. The received filling level envelope curve, however, does not only include the filling level echo that is representative of the current filling level, but often also includes unwanted or false echoes that are, for example, caused by multiple reflections or by reflections on parts mounted in the vessel.
In order to identify only the actual filling level echo in such a filling level envelope curve and to filter out the unwanted reflections, a pre-processing of the filling level envelope curve becomes necessary. During this pre-processing of the filling level envelope curve, the echoes are processed with the support of image processing methods such as filtering, averaging, edge recognition, selection and classification. The filling level envelope curve thus processed is subsequently examined and analyzed for echoes that are representative of the filling material, and for unwanted echoes. By means of the echoes thus processed, that, for example, include data such as location, amplitude and width of the echoes, a decision may then be made, which echo is representative of the current filling level and which is not. When an echo is detected as being representative of the current filling level, then the location of the analyzed echo corresponds to the searched filling level value.
Since a received filling level envelope curve may always include false echoes, as has already been explained, these must be safely recognized so as to prevent an erroneous filling level being determined from such an unwanted echo. A known criterion for assessing whether an echo is a filling level echo or an unwanted echo, consists in using always that echo as a filling level echo that has the largest amplitude. This criterion, however, must be judged uncertain, since an interfering object that is present in the signal propagation path and is closer to the receiver of the filling level measuring device than the current filling level, as a rule, will produce a higher echo than the filling level itself. Consequently, this criterion should never be applied alone but always and only in combination with further conditions.
From DE 42 23 346 A1, for example, an arrangement and a method is known for a contact-free distance measurement by means of pulse echo signals. For a more precise determination of signal transit times, this arrangement compares a pulse echo signal to signal patterns stored in a neuronal network. The signal transit time is supposed to be even then exactly determined when the pulse echo signal is considerably superposed by interfering echo signals. By means of parallel data processing techniques and the associative comparison of the received signal to acquired patterns that are stored in the neuronal network, it is by far better possible than it has been up to date to regenerate the hidden information and to determine with this very correct filling level data. The application of the neuronal-associative signal processing enables a complex holistic evaluation of the pulse-echo profile. Hereby, the measurement distance itself may be used as an intrinsic reference element, in that compensation magnitudes are derived from existing unwanted echoes.
From DE 42 34 300 A1, a filling level measuring method is known, in which the temporal shift of the useful echo caused by a changing signal transit time occurring during vessel filling or emptying processes is detected, and this criterion is evaluated so as to be able to distinguish the useful echo from unwanted echoes. Here, for distinguishing a useful echo from an unwanted echo, it is hence checked whether echo pulses exhibiting a continuous time shift are contained in subsequent signal progressions. Upon detection of such echo pulses, these are classified as being useful echoes. The basic idea of the procedure known from this consists in that signal transit times, upon reflection on the inner vessel walls, are stable with respect to time, so that the position of such interfering pulses is invariable even in repeated measurements within the received instruction. This time position stability within the reception profile normally also applies to the useful echo directly reflected from the filling material surface.
Other methods for filling level echo recognition refer to echo ratios received in the past, and compare these one by one to the echoes of the currently received filling level envelope curve. In these methods, the received echoes of an already received filling level envelope curve are stored in a memory so as to be able to compare them subsequently and individually to the data from a next filling level envelope curve. From EP 0 689 679 B1, a method is known that correlates currently received echoes with echoes already received in the past according to a difference value formation, and calculates from this, using a fuzzy evaluation unit, a probability for this echo to be a filling level echo. The problem of this procedure is, on the one hand, that this method is only suited for filtering out multiple echoes. After all and in addition, this method disclosed in EP 0 689 679 B1 only enables a comparison between echoes at two times. Observing the tendency of the filling level echo and forecasting the tendency, as well as an assignment to tendency ranges is not possible.
A further problem consists in that in a comparison between the newly received echoes and the echoes of already stored filling level envelope curves, assignment problems often arise, since the currently received echoes can change with time, although they are in each case always reflected by the same filling level surface as a reflector. Problems of this kind arise, for example, due to the formation of dust during the filling process or an after-slipping of filling material during emptying processes of bulk good vessels.
In order to ensure a safe filling level measurement, however, it is necessary that a filling level once recognized, is repeatedly recognized by means of currently received echoes, and that, for example, an interfering reflection is not erroneously evaluated as being representative of the filling level. If, for example, a filling level echo cannot be detected temporarily, this must be recognized so that an assignment is not made in which, for example, a false echo is identified as the filling level echo. This often problematic assignment of echoes from past filling level envelope curves to data of a current filling level envelope curve takes place in that, as has already been described, data of current echoes is compared to data of already received echoes. If in such a comparison, e.g. by means of a threshold value curve or a maximum search, a current echo corresponds to an already received echo, then it is assumed that these echoes correspond to each other, whereby the new echo is identified as a true echo. If the number of the echoes contained in an already received filling level envelope curve distinguishes with respect to the number of current echoes, then there exists the risk of assignment errors occurring. Likewise, there exists the risk of misassignment when several echoes arise in a narrow range.